We all know balancing your props is important, but what is the point of it if you do not balance your motors first?
Pretty simple one this. All you need in addition to you motor and RC gear is a laser pointer, electrical tape and something long and springy.
In this case I had a length of wood about so wide and 5mm thick. You could also try a standard ruler with just the motor mounted on it, and then you could use a few books instead of a clamp.
You could use any laser pointer, mine had been previously ripped apart. I rebuilt it with two AA batteries and taped it all together. The peg is to hold the button down.
Aim the laser pointer at something far away and white. If you are in a small room you could use a couple of mirrors to get extra distance.
Cycle the motor through its speed range. If it is unbalanced you will get a line or even a circle that will change shape with the rpm.
Stick some tape on the motor then see if it makes it better. Its hard to see here but there are two layers of electrical tape on the outside of this motor.
Keep moving the tape, trying different amounts until you get a nice steady dot throughout the speed range. I start with one piece of tape then keep moving around the axis of rotation until I find the sweet spot. Then if there is still a vibration there I start adding and removing the amount of tape on that spot. Moving the tape up and down on the axis can also make a difference.
Some more ideas.
I have not tried this yet but I guess you could balance the props while on the motors in same way. It might be wise however to add a string from the base of the motor to the ground to cancel out the thrust of the prop.
I have thought of doing this with the motors mounted on the quad. I am yet to try it but I would hang the copter with three strings with elastic bands inline somewhere to give it a bit of flex. Then put the controller in esc calibration mode, and unplug all but the motor you want to test.